PHP ver 5.4.24 does not support this function (it has not been defined). To get rid of this, you must copy image resource to new image created by function imagecreatetruecolor();
Example with image loaded from GIF file:
$image = imagecreatefromgif("path/to/gif/file.gif"); //create an image from GIF
$width = imagesx($image); //get width of source image
$height = imagesy($image); //get height of source image
$image2 = imagecreatetruecolor($width,$height); //create new image of true colors with given width and height
imagecopy($image2,$image,0,0,0,0,$width,$height); //copy source image to new one
header("Content-Type: image/jpeg"); //set header for JPG image
imagejpg($image2); //render JPg image into browser
imagedestroy($image); //free up memory
imagedestroy($image2);
